The present invention relates generally semiconductor devices and more particularly to a semiconductor device formed with an oxygen implant step.
Semiconductor devices are now found in almost all aspects of our lives. These products can be fabricated using any number of technologies including CMOS (complementary metal oxide semiconductor), bipolar, BiCMOS, and others. CMOS, as well as other MOS technologies, utilize field effect transistors (FETs). A field effect transistor includes two doped semiconductor regions, the source and drain, that are separated by a channel region. The conductivity of the channel region is actuated by a gate that typically overlies the channel.
To form a field effect transistor, a gate dielectric is deposited over a semiconductor substrate. A gate layer, typically of doped polysilicon is then deposited over the gate dielectric. The gate layer, and possibly the gate dielectric layer, are then etched to form the gate. After gate formation, the source and drain regions are implanted, using the gate as a mask to prevent the dopants from entering the channel region.
Many variations of this process are known. For example, a lightly doped drain (and/or source) can be formed. One way to form this region is to lightly dope the substrate using the gate as a mask. A sidewall spacer can then be formed over a portion of the lightly doped region. The more heavily doped source and drain regions can then be formed by implanting impurities using both the gate and the sidewall spacers as a mask. In another known process, the lightly doped drain can be formed by using a tilt angle implant process. Using an angle less than 90xc2x0 relative to the plane of the substrate, impurities can be located closer to the channel than the source/drain implant that is performed at substantially 90xc2x0.
The present invention provides a useful improvement for the formation of semiconductor devices. In particular, but certainly not exclusively, transistor devices used at the input and/or output of a semiconductor chip can benefit.
In a first embodiment, a transistor device includes first and second source/drain regions disposed in a semiconductor body and separated by a channel region. A gate overlies at least a portion of the channel region and is separated from the channel by a gate oxide. The gate oxide includes a thick portion beneath the gate adjacent a sidewall and has a substantially uniform thickness in other regions beneath the gate. In the preferred embodiment, the thick portion is at least about 1.2 times thicker than the substantially uniform thickness.
In another aspect, the present invention teaches a method of forming a semiconductor structure, which may or may not be a transistor device. In this embodiment, a silicon layer is formed and then patterned and etched to expose at least one sidewall. An oxygen bearing species, e.g., O2 or others, is then implanted into the sidewall of the silicon layer. The oxygen bearing species is implanted at an acute angle relative to the plane of the silicon layer.
When this method is applied to a transistor device, a number of advantages can be achieved. For example, the thick portion of the gate oxide that results provides for greater reliability. The oxygen implant yields a higher controllability of oxide corner rounding. The implant also provides a greater thickness and an easier adjustment with a low thermal budget than is possible using other techniques such as thermal oxidation of the gate stack. With the present invention, the implant angle, dose and energy can be optimized.